Solid state imagers generate electrical signals in response to light reflected by an object being imaged. Complementary metal oxide semiconductor (CMOS) imagers are one of several different known types of semiconductor-based imagers, which include for example, charge coupled devices (CCDs), photodiode arrays, charge injection devices and hybrid focal plane arrays.
Some inherent limitations in CCD technology have promoted an increasing interest in CMOS imagers for possible use as low cost imaging devices. A fully compatible CMOS imager technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital image capture applications. CMOS imagers have a number of desirable features, including for example low voltage operation and low power consumption. CMOS imagers are also compatible with integrated on-chip electronics (control logic and timing, image processing, and signal conditioning such as A/D conversion). CMOS imagers allow random access to the image data, and have lower manufacturing costs, as compared with conventional CCDs, since standard CMOS processing techniques can be used to fabricate CMOS imagers. Additionally, CMOS imagers have low power consumption because only one row of pixels needs to be active at any time during readout and there is no charge transfer (and associated switching) from pixel to pixel during image acquisition. On-chip integration of electronics is particularly desirable because of the potential to perform many signal conditioning functions in the digital domain (versus analog signal processing) as well as to achieve reductions in system size and cost.
Nevertheless, demands for enhanced resolution of CCD, CMOS and other solid state imaging devices, and a higher level of integration of imaging arrays with associated processing circuitry, are accompanied by a need to improve the light sensing characteristics of the pixels of the imaging arrays. For example, it would be beneficial to minimize, if not eliminate, the loss of light transmitted to individual pixels during image acquisition and the amount of crosstalk between pixels caused by light being scattered or shifted from one pixel to a neighboring pixel.
A significant source of photon reflection can occur at the junction of different media, each having a different refractive index. Photon reflection between two different media can be expressed by the following formula:
  R  =                    (                              n            1                    -                      n            2                          )            2                      (                              n            1                    +                      n            2                          )            2      where n1 and n2 are the refractive indices of the two media and R is the percentage of photons reflected at the junction of the two media.
Silicon and silicon oxide layers are required in many conventional CMOS photosensor structures because of the limitations of conventional CMOS technology and the high quantum efficiency of a crystallized silicon based photodiode.
With reference to FIGS. 1(a)-(c), which respectively illustrate a top-down view, a partial cross-sectional view and electrical circuit schematic of a conventional CMOS pixel sensor cell 100, when incident light 187 strikes the surface of a photosensor (photodiode) 120, electron/hole pairs are generated in the p-n junction of the photosensor (represented at the boundary of n-type accumulation region 122 and p-type surface layer 123 [FIG. 1(b)]). The generated electrons (photo-charges) are collected in the n-type accumulation region 122 of the photosensor 120. The photo-charges move from the initial charge accumulation region 122 to a floating diffusion region 110 via a transfer transistor 106. The charge at the floating diffusion region 110 is typically converted to a pixel output voltage by a source follower transistor 108 and then output on a column output line 111 via a row select transistor 109.
Conventional CMOS imager designs, such as that shown in FIGS. 1(a)-(c) for pixel cell 100, include a substrate 101 having a photosensor 120 and isolation regions 102. The floating diffusion region 110 is coupled to a transfer transistor gate 106′ of the transfer transistor 106. Source/drain regions 115 are provided for reset 107, source follower 108, and row select 109 transistors which have respective gates 107′, 108′, and 109′. A silicon dioxide layer 150 is typically formed over the substrate 101 to form a silicon-silicon dioxide stack, for example, as a protective layer.
A silicon/silicon dioxide stack 20 is shown in FIG. 2(a). A first layer 22 having a first refractive index, which corresponds to silicon dioxide layer 150 of FIG. 1(b), is formed on a second layer 21 having a second refractive index, corresponding to silicon substrate 101 of FIG. 1(b). However, formation of silicon dioxide on top of a silicon photodiode can lead to significant reflection at the junction of the two layers. Where the first layer is silicon dioxide (at or about n=1.45) and the second layer is silicon (at or about n=4), the stack 20 produces reflection R of about 22% of photons at the junction 23 of the first and second layers.
FIG. 2(b) shows a plot of the refractive index n of the stack of FIG. 2(a) relative to depth d. At the depth of junction 23, the refractive index n rises sharply from 1.5 to 4.0. FIG. 2(c) shows a plot of the total reflection R within the stack of FIG. 2(a) relative to depth d. At the junction 23, where n jumps from 1.5 to 4.0, the percentage reflection R spikes to 22%, which is undesirable. Referring back to FIG. 1(b), a significant quantity of photons is reflected at the junction between substrate 101 and silicon dioxide layer 150, and thus are not detected by the imager.
Accordingly, there is a need and desire for an improved solid state imaging device, capable of receiving and propagating light with minimal loss of light transmission to a photosensor. There is also a need and desire for improved fabrication methods for imaging devices that provide a high level of light transmission to the photosensor and reduce the light scattering drawbacks of the prior art, such as crosstalk and photon reflection.
There is also a need for improved display devices which utilize an array of photoemitters for light emission which also have improved light propagating properties.